Great cooler!
Go for the 25mm fan and make sure that there are no clearance issues between the fan and the RAM. If the RAM is too tall (with mini heatsinks) this must be checked out. The issue is usually with the 1st RAM slot (closest to the CPU). My Corsair Dominator RAM has a 1.0mm interference issue with the Prolimatech Megahalems Rev B cooler. If you run into interference issues, and have access to a milling machine, mill out a step on the fan housing to provide the necessary clearance.
As to your 2nd question - yes, you can install 2 fans in "push-pull" configuration, and the horizontal direction (not North-South) is preferable. Just my preference.
The cooler will work much better with 2 fans attached with clips.
Check the cooler base for flatness, and if required, do some polishing (sometimes referred to as lapping) to make the base flat. Increased flatness will reduce temps substantially. Lowered my idle temps 5 degs to 8 degs C by polishing my V8 cooler base.